Equipotential cathode



March 16, 1943. c. v. LITTON I EQUIPOTENTIAL CATHODE Filed Feb. 6, 1942 INVENTOR. CHARLES V. L/7'7'0/V ATTORNEY.

Patented Mar. 16, 1943 EQUIPOTENTIAL o-Arnonn Charles V. Litton, Redwood City, C'alif., assignor to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application February 6, 1942; Serial No. 429,747

6 Claims.

This invention relates to cathode arrangements andparticularly to cathodes of the unipotential type provided with separate heating elements.

The object of my invention is to provide a unipotential type of heated cathode, the temperature of which is maintained substantially uniform over the cathode emitting area.

Unipotential cathodes are in common use in vacuum tubes, arranged in the form of a surface such as a cylinder around an internally located heating element. The outer surface of the cathode is ordinarily coated with an active electron emitting substance so that when the heating element is heated, the cathode becomes heated and consequently active in emitting electrons. A disadvantage which has been encountered in the use of such cathode arrangements is that the ends of the cathode surface are not maintained at as high a temperature as the more centrally located portions, owing to conduction and radiation of the heat away from the ends. The result is that the central portions are more active in emitting electrons than the cooler end portions. This is a disadvantage particularly in tubes for high power use at ultra high frequencies, wherein it is desired to maintain a high and uniform rate of electron emission over the entire cathode surface.

In accordance with my invention, a construction is provided which maintains the temperature of the end portions so that they are substantially the same as the intermediate portions of the cathode. This is carried out by providing a heat conducting member which may constitute the cathode support itself so that heat will be conducted from the heating element to these edges at a sufiicient rate to offset the loss of heat at these edges.

Fig. l of the drawing shows a longitudinal section view taken at line II of Fig. 2, of a unipotential cathode construction arranged in relation to a heating element in a vacuum tube, according to my invention; and

Fig. 2 is a cross-section view of the structure taken at line 22 of Fig. 1.

In the drawing, walls Ill and II are glass sections of the envelope of a vacuum tube, only part of the section III being shown, and the rest of the tube envelope being omitted. The helix I2 is a cathode heating element which may be of a wellknown form, supported on rods I3 and M sealed through a pressed glass portion I5 of glass section II. The supports I3 and I4 are connected at the ends of the heater and act as electrical terminals for it.

Surrounding the heater I2 is a unipotential cathode I6, which is preferably cylindrical in form and may be. coated on its outer surface with an efficient electron emitting layer H, suchas thorium or a derivative thereof, so that under 1 the action of the heater-I2 the emitting surface I? will emit a large quantity of electrons outward toward an anode (not shown) which will ordinarily surround the cathode; This unipotential'cathode is supported by a collar-like electrical conducting member I8 around the heater and attached at-oneend to-the lower periphery 'of the cathode and at the other end to a suitable terminal support I9. The upper portion 2!) of support I8 preferably substantially surrounds the lower part of the heater I2 and may becylindrical or taper somewhat conically downward toward the heater supports. The upper portion of 2B is curved over a 2I to form a depending outer rim 22 which is afiixed around its periphery to the inner peripheral surface at the lower end of the oathode. The lower portion 23 of support I8 may be a conical member, or may consist of a plurality of struts or separated supports connecting with terminal I9. Terminal I9 is preferably made to form a metallic section of the tube envelope sealed between the glass sections I0 and II, for ready use as a cathode terminal.

In the operation of unipotential cathodes it has heretofore been found that there is a tendency for the production of less electron emission from the electron emitting surface at the ends of the cathode than at the central portion, the reason apparently being that the dissipation of heat from the ends causes the end portions to be at a lower temperature than the more centrally located portions, with the consequent decrease of emission at the cooler places. To prevent the cooling at the upper end of the cathode, heat shields 24 and 25 are provided, these being in the form of discs attached to the central rod I4 of the heater, and extending substantially across the upper open end of the cathode, so that heat from inside the oathode, which would otherwise pass out this end, is retained by the shields and caused to flow out toward the upper end 26 of the cathode to keep its temperature up.

Undesired cooling at the lower end 2! of the cathode is prevented by the operation of the specially arranged support I8 which receives heat from the heater, which heat flows around the curved part H and rim 22 to the lower part 21 of the cathode and thus supplies heat tothis lower portion sufiicient to compensate for the loss of heat which would otherwise cool it down below the temperature of the central portions of the oathode. By this arrangement, the temperature over the entire surface of the cathode can be maintained substantially uniform with a consequent uniformity of emission over this surface.

The invention is especially useful in electron discharge devices used for high power outputs at ultra high frequencies wherein it is especially desired that a very high rate of electron emission be provided uniformly from the entire cathode surface. A thoriated cathode as described, is well suited for supplying the required high emission, and the physical arrangement of the cathode and its attended parts according to this invention, enables a high degree of uniformity of emission to be obtained.

What is claimed is:

1. A unipotential cathode arrangement comprising a cylindrical unipotential cathode having an outer electron emitting surface, a support for the cathode having a portion of smaller diameter than said cathode and having a portion which flares from said smaller diameter portion to contact with an edge of the cathode, and a heating element located concentrically within the cathode and within the small diameter portion of said support.

2. In combination, a heating element, a unipotential cathode having an electron emitting surface around the heating element, and a metallic support for the cathode having a re-entrant shaped portion which enters an open end of the cathode and attaches around to the cathode at said open end, a portion of the surface of said support being located near the heating element to conduct heat to said edge.

3. In combination in a vacuum tube envelope, a heating element, a cylindrical unipotential cathode having an electron emitting surface around the heating element, a re-entrant shaped support entering an open end of the cathode and having an edge attached around the edge of the cathode, at least a part of the surface of said support near the re-entrant being located near the heating element to conduct heat to said edge, said support being attached to a metallic section of said envelope.

e. In combination, a heating element unipotential cylindrical cathode concentrically around the heating element, a disc-like heat shield at one of the open ends of said cathode, and a supporting means attached around the other open end of said cathode, said supporting means flaring in wardly from said end toward the heating element to conduct heat to said end.

5. In combination, a heating element, a uni potential cathode around said element having a thoriated outer surface adapted to emit electrons when said cathode is heated by said heating element, and a support for the cathode, said support being attached around one edge of the cathode, at least a portion of the surface of said support near said edge being located near the heating element to conduct heat to said edge.

6. A unipotential cathode arrangement comprising a unipotential cathode comprising tantalum cylinder with a thoriated electron emitting surface, an unthoriated metallic support for the cathode having a portion of smaller diameter than said cathode and a portion which flares outwardly from said smaller diameter portion to contact with an edge of the cathode and a heating element located within the cathode and within the small diameter portion of said support.

CHARLES V. LIT'ION. 

